It is often desired, for example in the medical field, to display objects that are not directly visible, such as organs. The conventional methods, such as radiography, echography, and so on, allow to obtain plane views of organs and a succinct idea of their aspect. More sophisticated techniques allow to reconstitute detailed cross-sectional views of the human body and synthesis images of organs from a plurality of cross-sectional views.
FIGS. 1 and 2 schematically illustrate two steps of an exemplary conventional technique for obtaining synthesis images of an object. These figures are very schematic and the various elements are shown at arbitrary scales.
FIG. 1 corresponds to a step for obtaining a set of linear radiological density projections of an object in order to obtain a cross-sectional view of the object in a plane The object 10 is examined along axes A.sup.1, A.sup.2, A.sup.3 . . . at various angles, positioned in the desired cross-section plane, and crossing a point 0 of object 10. For each axis, a sensor 12 acquires a profile approximately representing the density projection of object 10 along the considered axis A. Sensor 12 includes elements that are sensitive to excitations specific to the examination technique, for example to X-rays in an X-ray scanner. In the example of an X-ray scanner, a source SX provides an X-ray fan beam to the sensor through object 10, wherein the fan beam is parallel to the cross-section plane. The source SX is placed at a distance from the sensor to obtain substantially parallel rays in the cross-section plane. Each profile acquired along a respective axis A is then stored in a data processing system and corrected to account for the non-parallelism of the beams. Thus, a set of corrected profiles corresponding to radiological density projections of the object are stored in the memory.
FIG. 2 symbolically illustrates how to reconstitute a cross-sectional view of object 10 from density projections. The reconstitution method of FIG. 2 is a mathematical reconstitution usually referred to as a "filtered back projection".
Each projection is first processed by a deconvolution filter, for example a so-called "Shep and Logan" filter, for attenuating the edge effects or the distortions generated by the reconstitution method of the cross-sectional view. With each processed projection is associated a family of co-planar parallel lines. Each line of the family crosses a point of the processed projection and is assigned a coefficient representing the density of the point.
Each family is then directed to a point Oi along an axis corresponding to the respective examination axis A. Each point within the intersection surface of the families is assigned the sum of the density coefficients of the lines that cross this point. Within this surface, a cloud of points corresponding to a cross-sectional view of the examined object 10 is obtained. The density calculated for each point of this cloud substantially represents the density of the point corresponding to the object. The object definition provided by this cloud is all the best as the number of distinct axes A of examination is large.
By suitably processing this cloud of points, it is possible to display the cross-sectional view of the object and to bring out various areas by colors or different shades of gray. The areas that it is desired to bring out correspond, for example, to organs. An organ can be localized in the cross-sectional view with the various characteristics of the points corresponding to the organ, such as a dissimilar density, a dissimilar texture, and so on.
To realize a synthesis image of a full object, or of a part of an object, several consecutive cross-sectional views of the object are realized as explained above. The cross-sectional views are then superposed, and the missing points are interpoled to constitute the external surface of the desired portion. Then, an illumination of this external surface can be simulated to obtain a realistic rendering of its shape.